The said type of machines are usually given the name "circular machines for sweaters," in which knitting takes place with an assembly of needles, conventionally mounted in rotary needle beds, with dial and cylinder.
The needles are functionally actuated by cam sections which act on the said needles, leading them selectively along alternative paths appropriate for the pattern which is to be knitted, in such a manner that before the action of the cams, each needle has been selected in each section of cams, to follow any of the possible paths in the said section.
In each knitting section, the needles which have been selected to work take up yarn from a yarn guide which, in its turn, receives the yarn from a feed mechanism (such as is known from U.S. Pat. No. 2,006,232), which can be programmed at each revolution of the machine to deliver one or more out of six possible yarns.
In the said circular machines, the fabric is conventionally produced in the form of a continuous tube, with a specific number of spirals of stitches, each fragment of spiral knitted in one revolution of the machine receiving the designation of "pass."
The number of passes which is knitted per revolution of the machine coincides with the number of sections of the machine, when in the revolution in question all the sections are configured as "Jacquard sections," and knit a plain fabric, each pass being knitted by all of the needles of the machine.
However, the number of passes knitted in one revolution decreases when there are inoperative sections and/or when a "Jacquard pattern" of various colors is knitted, and also when each section is configured as a transfer.
In the machines without feeds, the passes join together with no break in continuity, since there exists no beginning or end in the needle beds, because the needles are uniformly distributed over the whole perimeter.
In contrast, in conventional machines with feeds, the passes have a beginning and an end which are physically predetermined, in such a manner that the beginning determines the first needle which receives yarn after the change of yarn delivered by the feed, and the end determines the last needle which knits before a change of yarn is produced, both needles always being the same ones.
Between the last needle and the first needle there is in the needle beds a zone without needles, called a "needle-free zone," which corresponds to the space necessary for the feed to have, conventionally, the means necessary to withdraw, cut, and retain the yarn which has been knitted, and to present, deliver and dispense the yarn which is to be knitted as a continuation.
Fabric is not formed in the said needle-free zone, because there are no needles in it, and the yarn appears in the same in the form of floats, that is, without forming stitches, so that the floats extend between the last needle of the preceding pass and the first of that which follows, which correspond to the last and first physical needles of the needle bed.
Immediately after the needle-free zone, there is what is conventionally termed the "initial zone," which begins with the first needle after the needle-free zone and is formed by a very small number of needles (generally less than twelve) which knit what is called the "initial welt."
In the said initial welt, the needles simultaneously knit two yarns, which are the salient yarn and the recessed yarn, so that the process of fabric formation is given continuity, avoiding the production of a discontinuity when the yarn changes (change of feeds).
Before the needle-free zone, there is what is conventionally termed the "end zone," which is programmed for the needles to knit according to a selection which is suitable for forming the final welt.
The initial zone, needle-free zone and end zone form what is called the "change zone," while the initial and final welts give continuity to the knitted tube, counteracting and eliminating the discontinuity of the needle-free zone, so that to all intents and purposes the fabric is produced physically in the form of a tube, with the consistency and behavior of a homogeneous tube.
The said characteristics permit the actions of stretching, calendering, and rolling up the fabric, which can be effected in the same manner as in a circular machine without feeds. These three actions are exerted by the folding device of the machine, which receives the fabric produced in the form of a tube in the needle beds, and which calenders, flattens, and folds it in two, and which continuously receives it and rolls it up.
In flat bed knitting machines, the fabric is produced in a flat form, and there thus exist a physical beginning and end of the stroke, which permits the production of a fabric with variable width and/or with varied effects.
On the other hand, the effectiveness of the folder in flat bed machines is less, because of the discontinuity of the fabric (which has a beginning and end in each pass), aggravated by the transverse elasticity characteristic of knitted fabrics.
The advantages of variable width and structured fabrics, which the flat bed machines possess with respect to the circular machines, are distinguished by the following:
(a) The variable width consists of knitting with the exact number of needles which are necessary for obtaining the precise fabric width for making the garment which is to be knitted. PA1 (b) Structured fabrics are up to now an exclusive feature of flat bed machines, and are obtained by the controllable displacement of the relative position of the needle beds.
This feature eliminates waste in making up, because in each case it is the necessary knitted width which is produced, while in conventional circular machines this arrangement is not possible, due to the physical configuration, because the circular continuity of the needle bed favors and determines the continuity and magnitude of the width of the tube of fabric which is produced.
However, the discontinuity of the change zone in machines with feeds does not permit the said feature, because the first and last needles are always invariably determined.
In one position, each needle of a needle bed is situated between two needles, which are always the same, of the opposed needle bed, in such a manner that in conventional circular machines the said position is unalterable and never changes, and the stitches of the knitted fabric are always produced in the same order; and in the case of stitch transfers, they are always produced between the same pairs of needles.
In contrast, in flat bed machines, the discontinuity of the process of knitting permits changing the relative position of the needle beds in the dead times which occur at the end of each stroke, and the order in which the stitches are produced can be varied and stitches transferred to different needles in different strokes.
This feature is facilitated in flat bed machines by the obligatory discontinuity of the knitting process at the end of each stroke; however, on the other hand, there is a reduction of the productive capacity, because the dead time always occurs, independently of whether or not the relative position of the needle beds changes in the stroke which develops.